The ovarian luteal cell in both the human and the dog possesses a number of distinct subcellular compartments that are oriented with a high degree of consistency towards the pericapillary space. Such distinct compartmentalization has not been described for other species studied thus far. While much is known about the biochemical pathways leading to the production of progesterone, and even the general class of organelles responsible, there is almost nothing known about the polarity of this steroidogenic process. Of particular importance, is that neither the site of progesterone "storage" and release, nor the subcellular fate of the trophic hormones has been identified. A number of factors have impeded the solution of these problems: e.g., the lack of a good, highly compartmentalized luteal cell model, the diffusible nature of the steroid molecule itself, the lack of sensitivity of available cytochemical techniques, and the limitations imposed by the technique of normal transmission electron microscopy. However, recent advances in ultrastructural cytochemistry, the development of high voltage electron microscopy, and the discovery of a highly polarized "human-like" cell model suggest, that a re-evaluation of the compartmentalization of various luteal cell functions is warranted. Experiments utilizing slices of dog luteal tissue in vitro in combination with cytochemical and biochemical tracer techniques will be conducted to: 1) determine exactly where on the cell tropic hormone binding occurs and follow the fate of the receptor-hormone complex with time; 2) analyze the function of the Golgi apparatus; 3) follow morphologically, and in a step-wise manner (through the use of specific inhibitors), the movement of radioactively labeled steroid precursors through the synthesis, storage and release of progesterone; 4) analyze the role of microtubules and microfilaments in progesterone secretion; and, 5) determine the functional role and identity of the various heterogeneous subcellular granules.